Canvas accessibility draft proposal

So, I am listening to all this and would like to make a draft proposal for
the HTML working group meeting this week (Paul Cotton made a request):

1. Modify the canvas element to support aria-activedescendant where the id
refers to a valid id in a shadow DOM.
2. The canvas element shall support zero or more child access elements
representative of alternative views. The access element shall have a mode
attribute. Whose values are based on a subset of the IMS Global Consortium
Access for All Meta Data V3 default. The default mode would be
representative of the shadow DOM. In many instances a more usable solution
may require an alternative.
3. The mode content attribute values for access would be:
- default - representing the shadow DOM for the complex visual rendering of
<canvas>
- textual - Here we could place a long description or an alternative
accessible UI renderable using HTML 5 standard markup - including ARIA.
- auditory - an audio alternative. Some users may prefer this
- visual - This is really an alternative for audio but it would address
things like signLanguage

Note: access for all also supports "tactile" and "olfactory". I would
recommend ignoring those for now. These could added in future releases of
HTML.

4. The access element would include a type attribute for which we should
take a subset of the list (from AccessForAll): audioDescription, caption,
e-book, signLanguage, highContrast, transcript, alternativeText,
longDescription, haptic}

The e-book may be rendered with a plug-in. We can choose a subset of these
if necessary. At this point this is for discussion purposes.

5. View Selection should be based on a user preference in the browser based
on the mode and type preference supported in HTML 5. If nothing is set, the
shadow DOM is used for the accessibility mapping when provided. The user
agent should select the first view that fits the user's request. Platform
accessibility API may also be used to configure the view selection. Note:
is no alternative view is provided to meet the preferences the view reverts
back to the canvas + shadow DOM view. The shadow DOM is also optional.

6. The canvas tag shall include a script method to set the bounding
rectangle for a given valid id in the shadow DOM. This would allow
the browser to provide the bounding rectangle for the id in the shadow DOM
when it receives focus. The rectangle should be relative to the location of
the canvas element and would be converted to screen coordinates in the
accessibility API and would allow the canvas to move without changing the
location of the shadow DOM element. This will allow screen magnifiers to
determine the visible focus. (Ideally it would be nice to also do this via
XPath statements vs. id)
7. All allowable HTML elements in the shadow DOM shall support the tabindex
content attribute and ARIA attributes.
8. The shadow DOM should include all renderable HTML elements save standard
input controls. The reasoning being is these
controls are user agent managed in terms of the keyboard and would likely
conflict with canvas keyboard management.


The format would be along the following lines:
<canvas aria-activedescendant="foo">
   <access mode="default">  /*Since canvas is visual this is the default
mode*/
     <div role="toolbar">
        <div role="tab" tabindex="-1" id="foo">:
        </div>
     </div>
   </access>
   <access mode="textual"> /*Here we could place a long description or an
alternative aria-enabled widget*/
   </access>
   <access mode="auditory">
   </access>
   <access mode="tactile"> /*This is in access for all and we could lose it
for now for obvious reasons*/
   </access>
   <access mode="olfactory"> /*This is in access for all and we could lose
it for now for obvious reasons*/
   </access>
   <access mode="visual" type="signlanguage">
   </access>
</canvas>

Rich Schwerdtfeger
Distinguished Engineer, SWG Accessibility Architect/Strategist